It is essential to increase a relative speed between the steel plate and the cooling water for increasing the cooling speed of the same plate and thereby improving the efficiency of the hardening operation. In a known cooling procedure, such a feature has been realized by injecting a pressurized cooling water onto the surface of the steel plate through a plurality of nozzles.
However, such a cooling procedure inevitably requires a large and heavy installation for the pressurized cooling water and a large quantity of the cooling water requiring a high installation and operational cost. Furthermore, it requires water supplying and exhausting pipe lines of a large diameter, necessitating a large installation space and increasing the height and length of the hardening device. In addition, the large and thereby heavy pipe lines and heavy contents therein require an additional reinforcement of the supporting members such as supporting frame and the like, while the pipe line of a large size increases the pitch between pipe lines provided in parallel, thus making it difficult to increase the density of injected cooling water and to maintain a required capability of cooling the steel plate. Furthermore, a problem such as clogging of nozzles tends to result in uneven hardening of the steel plate, and increases the time and labor required for eliminating the clogging of the nozzles.
A cooling device for steel plate has been disclosed in Japanese Patent Publication No. 11247/1978. In this device, there are provided a number of roller pairs supporting the steel plate on the upper and lower sides of the steel plate and a plurality of upper and lower enclosures arranged symmetrically for encasing a pair of the rollers and a part of the steel plate adjacent to the pair of rollers between the upper and lower enclosures. The widthwise and lengthwise ends of the enclosures are closed. Cooling water supply tubes and cooling water exhaust tubes are connected alternately to the enclosures. Narrow spaces provided between the enclosure and the steel plate provide passages of the cooling water along which the heat of the steel plate is dissipated. In this cooling device, however, since the rollers and the steel plate are encased in each enclosure with narrow spaces formed between the enclosure and the steel plate being utilized for passages of the cooling water, a probability of the passages being clogged still exists. When the clogging of the passages occurs, the cooling effect of the cooling device is thereby substantially reduced.
It is an object of the present invention to provide a continuous hardening device of steel plate wherein the above described difficulties of the conventional devices are substantially eliminated, and a desired cooling capability can be maintained regardless of its small-size construction and a reduced consumption of cooling water.